1. Field of the Invention
The present invention relates generally to the field of data transmission using cable modems in a cable television plant. More specifically, the invention relates to transmitting data upstream to a headend by locating cleaner and more reliable data carriers.
2. Discussion of Related Art
The cable TV industry has been upgrading its signal distribution and transmission infrastructure since the late 1980s. In many cable television markets, the infrastructure and topology of cable TV systems now include fiber optics as part of its signal transmission component. This has accelerated the pace at which the cable industry has taken advantage of the inherent two-way communication capability of cable systems. The cable industry is now poised to develop reliable and efficient two-way transmission of digital data over its cable lines at speeds orders of magnitude faster than those available through telephone lines, thereby allowing its subscribers to access digital data for uses ranging from Internet access to cable commuting.
Originally, cable TV lines were exclusively coaxial cable. The system included a cable headend, i.e. a distribution hub, which received analog signals for broadcast from various sources such as satellites, broadcast transmissions, or local TV studios. Coaxial cable from the headend was connected to multiple distribution nodes, each of which could supply many houses or subscribers. From the distribution nodes, trunk lines (linear sections of coaxial cable) extended toward remote sites on the cable network. A typical trunk line is about 10 kilometers long. Branching off of these trunk lines were distribution or feeder cables (40% of the system's cable footage) to specific neighborhoods, and drop cables (45% of the system's cable footage) to homes receiving cable television. Amplifiers are provided to maintain signal strength at various locations along the trunk line. For example, broadband amplifiers are required about every 2000 feet depending on the bandwidth of the system. The maximum number of amplifiers that can be placed in a run or cascade is limited by the build-up of noise and distortion. This configuration, known as tree and branch, is still present in older segments of the cable TV market.
With cable television, a TV analog signal received at the headend of a particular cable system is broadcast to all subscribers on that cable system. The subscriber simply needed a television with an appropriate cable receptor to receive the cable television signal. The cable TV signal was broadcast at a radio frequency range of about 50 to 860 MHz. Broadcast signals were sent downstream; that is, from the headend of the cable system across the distribution nodes, over the trunk line, to feeder lines that led to the subscriber's home or premises. However, the cable system did not have installed the equipment necessary for sending signals from subscribers to the headend, known as return or upstream signal transmission. Not surprisingly, nor were there provisions for digital signal transmission either downstream or upstream.
In the 1980s, cable companies began installing optical fibers between the headend of the cable system and distribution nodes (discussed in greater detail in FIG. 1 below). The optical fibers reduced noise, improved speed and bandwidth, and reduced the need for amplification of signals along the cable lines. At many locations, cable companies installed optical fibers for both downstream and upstream signals. The resulting system is known as a hybrid fiber-coaxial (HFC) system. Upstream signal transmission was made possible through the use of duplex or two-way filters. These filters allow signals of certain frequencies to go in one direction and signals having different frequencies to go in the opposite direction. This new upstream data transmission capability allowed cable companies to use set-top cable boxes and allowed subscribers pay-per-view functionality, i.e. a service allowing subscribers to send a signal upstream through the cable system to the headend indicating that they want to see a certain program.
In addition, cable companies began installing fiber optic lines into the trunk lines of the cable system in the late 1980s. A typical fiber optic trunk line can be up to 80 kilometers long, whereas a typical coaxial trunk line is about 10 kilometers long. Prior to the 1990s, cable television systems were not intended to be general-purpose communication mechanisms. Their primary purpose was transmitting a variety of television signals to subscribers. Thus, there had to be one-way transmission paths from a central location, known as the headend, to each subscriber's home, delivering essentially the same signals to each subscriber. HFC systems run fiber deep into the cable TV network offering subscribers more neighborhood specific programming by segmenting an existing system into individual serving areas having between 100 to 2,000 subscribers. Although networks using exclusively fiber optics would be optimal, present cable networks equipped with HFC configurations are capable of delivering a variety of high bandwidth, interactive services to homes for significantly lower costs than networks using exclusively fiber optic cables.
FIG. 1 is a block diagram of a two-way HFC cable system utilizing a cable modem for data transmission. It shows a headend 102 (essentially a distribution hub) which can typically service about 40,000 subscribers. Headend 102 contains a cable modem termination system (CMTS) 104 needed when transmitting and receiving data using cable modems. Headend 102 is connected through pairs of fiber optic lines 106 (one line for each direction) to a series of fiber nodes 108.
Each headend can typically support up to 80 fiber nodes. Pre-HFC cable systems used coaxial cables and conventional distribution nodes. Since a single coaxial cable was capable of transmitting data in both directions, one coaxial cable ran between the headend and each distribution node. In addition, because cable modems were not used, the headend of pre-HFC cable systems did not contain a CMTS. Each of the fiber nodes 108 is connected by a coaxial cable 110 to duplex filters 112 which permit certain frequencies to go in one direction and other frequencies to pass in the opposite direction (frequency ranges for upstream and downstream paths are discussed below). Each fiber node 108 can normally service about 500 subscribers, depending on the bandwidth. Fiber node 108, coaxial cable 110, two-way amplifiers 112, plus distribution amplifiers 114 along trunk line 116, and subscriber taps 118, i.e. branch lines, make up the coaxial distribution system of an HFC system. Subscriber tap 118 is connected to a cable modem 120. Cable modem 120 is, in turn, connected to a subscriber computer 122 or other appropriate device.
As briefly mentioned above, recently it has been contemplated that HFC cable systems can be used for two-way transmission of digital data. The data can be Internet data, digital audio data, or digital video data, in MPEG format, for example, from one or more external sources 100. Using two-way HFC cable systems for transmitting digital data is attractive for a number of reasons. Most notably, they provide up to a thousand times faster transmission of digital data than is currently possible over telephone lines. However, in order for a two-way cable system to provide digital communications, subscribers must be equipped with cable modems, such as cable modem 120. With respect to Internet data, the public telephone network has been used, for the most part, to access the Internet from remote locations. Through telephone lines, data is typically transmitted at speeds ranging from 2,400 to 56,600 bits per second (bps) using commercial (and widely used) data modems for personal computers. Using a two-way HFC system as shown in FIG. 1 with cable modems, data can be transmitted at speeds of 10 million bps, or more. Table 1 is a comparison of transmission times for transmitting a 500 kilobyte image over the Internet.
TABLE 1Time to Transmit a Single 500 Kbyte ImageTelephone Modem (28.8 kbps)6-8minutesISDN Line (64 kbps)1-1.5minutesCable Modem (10 Mbps)1second or less
Furthermore, subscribers can be fully connected twenty-four hours a day to services without interfering with cable television service or phone service. The cable modem, an improvement of a conventional data modem, provides this high speed connectivity and, therefore, is instrumental in transforming the cable TV system into a full service provider of video, voice and data telecommunications services.
As mentioned above, the cable TV industry has been upgrading its coaxial cable systems to HFC systems utilizing fiber optics to connect headends to fiber nodes and, in some instances, using them in the trunk lines of the coaxial distribution system. In way of background, optical fiber is constructed from thin strands of glass that carry signals longer distances and have a wider bandwidth than either coaxial cable or the twisted pair copper wire used by telephone companies. Fiber optic lines allow signals to be carried much greater distances without the use of amplifiers (item 114 of FIG. 1). Amplifiers degrade the signal quality and can be expensive to maintain. Thus, coaxial distribution systems that use fiber optics have much less need for amplifiers. In addition, amplifiers are typically not needed for fiber optic lines (item 106 of FIG. 1) connecting the headend to the fiber nodes.
In cable systems, digital data is carried over radio frequency (RF) carrier signals. Cable modems are devices that convert a modulated RF signal to digital data (demodulation) and converts the digital data back to a modulated RF signal (modulation). The conversion is done at two points: the subscriber's home by a cable modem and at a CMTS located at the headend. The CMTS converts the digital data to a modulated RF signal which is carried over the fiber and coaxial lines to the subscriber premises. The cable modem then demodulates the RF signal and feeds the digital data to a computer. On the return path, the operations are reversed. The digital data is input from a computer to the cable modem which converts it to a modulated RF signal. Once the CMTS receives the RF signal, it demodulates it and transmits the digital data to an external source.
As mentioned above, cable modem technology is in a unique position to meet the demands of users seeking fast access to information services, the Internet and business applications, and can be used by those interested in cable commuting. Not surprisingly, with the growing interest in receiving data over cable network systems, there has been a sharper focus on performance, reliability, and improved maintenance of such systems. Consequently, cable companies are in the midst of a transition from their traditional core business of entertainment television programming to being full service providers of video, voice and data telecommunication services. Among the elements that have made this transition possible are technologies such as the cable modem.
A problem common to all upstream data transmission on cable systems, i.e. transmissions from the cable modem in the home back to the headend, is ingress noise which lowers the signal-to-noise ratio of an upstream frequency channel. Ingress noise can result from numerous internal and external sources. Sources of noise internal to the cable system may include cable television network equipment, subscriber terminals (televisions, VCRs, cable modems, etc.), intermodular signals resulting from corroded cable termini, and core connections. Significant sources of noise external to the cable system include home appliances, welding machines, automobile ignition systems, and radio broadcasts, e.g. citizen band and ham radio transmissions. All of these ingress noise sources enter the cable system over the coaxial cable line, which acts essentially as a long antenna. Ultimately, when cable systems are entirely optical fiber, ingress noise will be a far less significant problem. However, until that time, ingress noise is and will continue to be a problem with upstream transmissions.
The portion of bandwidth reserved for upstream signals is normally in the 5 to 42 MHz frequency range in the United States, and 5 to 65 MHz in Europe. Portions of this upstream band may be allocated for set-top cable boxes, pay-per-view, voice data, and other services provided over the cable system. Thus, cable modems may only be entitled to some fraction of the entire upstream channel. A cable modem session may be allotted a “sub-band” typically between 200 KHz to 3.2 MHz wide within the fraction. This sub-band is referred to as a cable modem's “allotted band slice” of the entire upstream frequency range (5 to 42 MHz, or 5 to 65 MHz). However, problems arise because this portion of the spectrum—from 5 to 42 MHz in the United States—is particularly subject to ingress noise and other types of interference. Thus, cable systems offering two-way data services must be designed to operate given frequent ingress noise interference which effects data transmission reliability and accuracy.
As noted above, ingress noise, typically narrow band, e.g., below 100 KHz, is a general noise pattern found in cable systems. Upstream channel noise resulting from ingress noise adversely impacts upstream data transmission by reducing data throughput and interrupting service thereby adversely affecting performance and efficient maintenance.
When a particular group of contiguous sub-bands or allotted band slices, referred to as an “upstream frequency channel” or “frequency channel,” reaches an unacceptable signal to noise ratio, the CMTS begins searching for a cleaner, unused frequency channel for the upstream signal. In other words, when a frequency channel gets too noisy, the CMTS will automatically locate an alternative frequency channel. A frequency channel is used by a group of cable modems (the grouping typically based on physical location) to transmit signals upstream to the headend.
However, changing frequency channels for a group of modems requires significant overhead in terms of processing at the headend by the CMTS and cable plant signal traffic. Cable modems in the cable plant can be divided into groups (or subscriber areas) in which cable modems in each group share the same upstream frequency channel. This is possible by using, for example, time division multiplexing, a technique known in the art. In addition, the other frequency channels may not result in an improved signal to noise ratio if there is general ingress noise effecting the entire upstream band in the cable plant. If there is a major source of noise in the external or internal environments to the cable plant spanning a wide frequency spectrum, the headend can continually switch upstream frequency channels and still not result in any significant improvement in signal to noise ratio. In this situation and in other less extreme situations it may be better to continue using the current frequency channel even if its noise level is above a certain threshold level. In this case, transitioning to another frequency channel would be undesirable. The overhead in traffic on the cable plant resulting from the CMTS having to inform each cable modem to change frequencies can be high, typically requiring up to ten minutes. It is preferable to minimize using the fiber and coaxial lines for signaling frequency changes and indicating telemetry status of the cable modems. It would be preferable to continue using the same frequency channel so that the CMTS would not have to send out initial maintenance messages to all the cable modems and not have to receive ranging request messages from those cable modems in order to allot new timing marks to the cable modems. This process is better avoided if possible, especially if there is no guarantee that an alternative frequency channel would result in a better signal to noise ratio.
Therefore, it would be desirable to be able to detect or identify a sub-bandwidth within a currently used upstream frequency channel that has an acceptable noise level and can be used to transmit signals upstream. It would be desirable to identify this sub-bandwidth in the frequency channel while the frequency channel is in use. If transitioning to another frequency channel, it would be desirable to know of this sub-bandwidth before transitioning. It would also be desirable to take into account certain modulation schemes when detecting the sub-bandwidth, such as QPSK and QAM16, as instructed by the DOCSIS standard for cable modems.